Human plasma alpha-cysteine proteinase inhibitor. Purification by affinity chromatography, characterization and isolation of an active fragment.

Human plasma alpha-cysteine proteinase inhibitor (alpha CPI) was purified by a two-stage method: affinity chromatography on S-carboxymethyl-papain-Sepharose, and high-resolution anion-exchange chromatography. The protein was obtained as a form of Mr about 64 000 and material of higher Mr (about 100 000). In sodium dodecyl sulphate/polyacrylamide-gel electrophoresis with reduction, both forms showed a major component of Mr 64 000. An antiserum was raised against alpha CPI, and 'rocket' immunoassays showed the mean concentration in sera from 19 individuals to be 35.9 mg/dl. Both low-Mr and high-Mr forms of alpha CPI were confirmed to be sialoglycoproteins by the decrease in electrophoretic mobility after treatment with neuraminidase. alpha CPI was shown immunologically to be distinct from antithrombin III and alpha 1-antichymotrypsin, two serine proteinase inhibitors from plasma with somewhat similar Mr values. alpha CPI was also distinct from cystatins A and B, the two intracellular low-Mr cysteine proteinase inhibitors from human liver. Complexes of alpha CPI with papain were detectable in immunoelectrophoresis, but dissociated to free enzyme and intact inhibitor in sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. The stoichiometry of binding of papain was close to 1:1 for both low-Mr and high-Mr forms. alpha CPI was found to be a tight-binding inhibitor of papain and human cathepsins H and L (Ki 34 pM, 1.1 nM and 62 pM respectively). By contrast, inhibition of cathepsin B was much weaker, Ki being about 35 microM. Dipeptidyl peptidase I also was weakly inhibited. Digestion of alpha CPI with bromelain gave rise to an inhibitory fragment of Mr about 22 000, which was isolated.